A well made endless string will be faster than a poorly made flemish, and vice-versa, if the materials, strand count, serving size and length, etc. are all the same.
I know Jack Harrison and Mike Fedora are well respected as bowyers, but I haven't heard of either one competeing. Many of the top 3-D archers use flemish strings, while others use endless.
If you are picking up 8 fps using an endless over a flemish, there's some big differences in the string (different material, different strand count, different serving size, etc.) or the faster one is well made and the slower one is poorly made......or all of the above. The same applies if you are seeing that kind of speed gain with flemish over endless.
Sure, Olympic archers used endless strings...I've heard a few reasons why, the most logical being that endless strings are easier to duplicate from one builder to the next. Personally, I think it's a mental thing. All I can do here is offer an opinion, because I have never heard the reason straight from the horse's mouth.
I make a lot of strings, and I make both flemish and endless. I'm a hunter who shoots a lot of 3-D. My preferance, as a hunter, is flemish.
That's what it boils down to, if you compare apples to apples--preferance.
Chad